A search engine uses a ranking model to respond to a user's query, typically by generating search results in the form of a ranked list of search result items. In many cases, the ranking model determines the ranking of items in a user-agnostic manner. As such, the search engine will deliver the same search results to two distinct users who submit the same query. This behavior is satisfactory in many cases. However, the search engine can be expected to offer less optimal results for any user who has a specialized search intent that is not adequately addressed by the search engine's ranking model.
To address this issue, the research community has proposed numerous techniques for personalizing the behavior of a search engine based on the assessed characteristics of individual users. However, there is room for improvement in this field of research.